Plastics are typically made from non-renewable petroleum resources and are often non-biodegradable. In the United States, plastics are produced in amounts exceeding 115,000 million pounds annually. Plastics are used in many industries to form products for sale in both industrial and residential markets. In industrial markets, plastics are used to form packaging, insulation, construction products, etc. In residential markets, plastics are used to form bottles, containers, and the like.
Plastics such as polyethylene terephthalate (PET), high density polyethylene (HDPE), and polyvinyl chloride (PVC), have commonly accepted Recycling Codes of from 1 to 3, respectively, as developed by the American Plastics Council. These aforementioned plastics are more widely recycled and re-used than many other types of plastics. However, plastics such as polyethylenes having Recycling Codes of 2, 4, and 7, polypropylene having a Recycling Code of 5, and polystyrene having a Recycling Code of 6, can also be recycled. Yet, recycling efforts for polyethylenes, polypropylene, and polystyrene have not been maximized.
Only a small fraction of the plastics produced each year are recycled and re-used. To ease in recycling, the plastics are usually crushed, melted, and/or broken down. Plastics that are not recycled and re-used present potential environmental pollution risks when discarded, are not utilized for energy or raw materials, and contribute to an increased reliance on non-renewable petroleum resources. Traditionally, plastics are recycled according to one of two methods including open- and closed-loop recycling. Closed-loop recycling involves using the plastic as an input to make the same product again. Open-loop recycling involves using the plastic as an input to make other products. For example, open-loop recycling may be used to form diesel fuel using the plastic as an input. However, neither of these methods are particularly efficient because of the complexities involved in processing plastics of different colors, textures, and consistencies and producing other products.
One particular type of open loop recycling includes decomposition of a plastic by heating, in the absence of a catalyst, to reverse polymerize the plastic and form monomers. After the plastic is decomposed, the monomers can then be used in a variety of manufacturing or commercial processes. Traditionally, this decomposition through heating forms monomers having an inconsistent and/or unpredictable number of carbon atoms, while leaving much of the plastic unusable. Formation of monomers having unpredictable numbers of carbon atoms inhibits the monomers from being effectively recycled into other products.
Another particular type of open-loop recycling includes catalytic cracking, which improves on the decomposition of plastic by heating alone. As is known in the art, catalytic cracking involves reverse polymerizing a plastic, in the presence of a catalyst, to form monomers. Traditionally, the catalysts used in catalytic cracking procedures include classic Lewis acids such as AlCl3, metal tetrachloroaluminates, zeolites, superacids, gallosilicates, metals on carbon, and basic oxides. However, many of these catalysts are ineffective in selectively cracking the plastics to form specific monomers. Although traditional catalytic cracking is more efficient in forming monomers than simple decomposition of plastics through heating alone, many of these traditional catalysts still form monomers having an inconsistent and/or unpredictable number of carbon atoms and still leave much of the plastic unusable and un-cracked. Accordingly, there remains an opportunity to develop an improved method for recycling plastics.